We were successful in ascertaining the presence of T-23-I and T-23-II with ansamycin skeleton having anti-tumor activities in the fermentation product of a new strain Streptomyces rishiensis T-23 (deposited at "Fermentation Research Institute, Agency of Industrial Science & Technology, MITI(Japan)" under the deposit No. FERM P-6141), and previously proposed the processes for the production of T-23-I and T-23-II as disclosed in Japanese Patent Application Nos. 189237/1981 and 189238/1981 (laid open to public as L-O-P Publication Nos. 94393/1983 and 92662/1983, respectively). A U.S. patent application based on these Japanese patent applications was issued as U.S. Pat. No. 4,521,339 on June 4, 1985. According to the abovementioned processes, the T-23 strain was cultured according to the conventional method for the culture of Actinomyces strain. The culture broth obtained was divided into mycelia and a supernatant. An active fraction was extracted with acetone-water from the mycelia. The extract containing the active fraction was filtered. The filtrate was passed through a nonionic exchange resin to effect absorption of the active fraction thereon, followed by elution with a solvent such as acetone or a lower alcohol. The active fraction was also extracted with an organic solvent directly from the supernatant. The two extracts obtained were combined, and the organic solvent was removed from the mixture to give an aqueous phase which was extracted with a water-immiscible solvent such as chloroform or ethyl acetate and concentrated. Thereafter, the concentrate was charged with an aliphatic hydrocarbon solvent to precipitate the active fraction. The active fraction was adsorbed onto a silica gel column. After washing with benzene, the column was eluted with benzene-acetone (4:1) to obtain a T-23-I containing solution and with benzene-acetone (7:3) to obtain a T-23-II containing solution.